This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 00112068.2 filed in Europe on Jun. 5, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to a process of repairing a coated component according to the preamble of claim 1.
Most turbine components are coated for protection from oxidation and/or corrosion with, for example, a MCrAlY coating (base coat) and some are also coated with a thermal barrier coating (TBC) for thermal insulation. The demands of operation of the parts in a gas turbine often lead to the degradation of the coating before the structural integrity of the underlying part itself is degraded. Hence, the base coat and TBC must be removed and reapplied. Such processes are known from EP-A2-813 930, EP-A1-298 309 or U.S. Pat. No. 5,728,227
The coatings must be replaced because during service they degrade by forming protective aluminium and/or chromium oxides on the surface, which periodically spall off and must be replaced by fresh Al and/or Cr from the coating. Hence, Al and/or Cr diffuses from the interior of the coating towards the coating surface to continually replenish the protective oxides. It is known that the level of degradation of a coating (i.e. the remaining life) can be characterised by the amount of Al and/or Cr, left in the coating compared with the amount first present in the originally applied coating. It is also known that turbine blades and other components have only local areas of extremely high surface temperature during operation, whereas the rest of the component surface has a moderate temperature. This means that the environmentally protective coatings are degraded by far the most in these local areas of high temperature, which constitute about 5 to 20% of the total surface area of the whole component, and only moderately over the rest of the surface area. For older gas turbine engines where components operate at a low temperature, it is widely practised to locally replace the depleted coatings (MCrAlY) in the local hot areas, but leave the rest of the coating alone since it is not yet depleted in Al and/or Cr to the point that it cannot survive another inspection interval of the engine operation
However, modern gas turbines operate at higher temperatures, where local hot spots on the blades completely deplete Al from the MCrAlY coatings, and partial depletion of Al from the coating over the rest of the blade is such that they usually can not survive another inspection interval. Therefore the entire coating must be stripped and replaced. The stripping of the coating involves a treatment with very aggressive acid which also removes some of the base material of the blade, may lead to weakening of the material at the surface, is expensive and time consuming. Re-coating of the blade is also expensive, and both operations have a certain scrap rate due to problems in the process.
It is an object of the present invention to find a method of restoring enough Al and/or Cr to the partially depleted MCrAlY-coating on the cooler parts of the turbine blades so that it could survive a complete inspection interval of engine operation, while only locally repairing the xe2x80x9chot spotsxe2x80x9d which are completely depleted and beyond such restoration due to the excessive coating spallation.
According to the invention a process was found of repairing an article with a MCrAlY-coating after use of the article in a high temperature environment where the MCrAlY-coating is repaired locally and the article is aluminised and/or chromised on the surface of the article on top of the MCrAlY-coating
This method saves both time and investments costs because stripping and recoating using usual plasma spraying would be unnecessary. On the other hand is it possible to replenish the amount of Al and Cr in the depleted surface of the article in an easy way.
The areas requiring local repair of the MCrAlY coating are by definition subject to the highest levels of depletion of Al and/or Cr. Associated with this depletion will be a significant thickness of depleted (non-functioning) MCrAlY coating and also of oxide scale. If the repair MCrAlY coating material is to properly bond to the substrate, all of the oxide scale must be removed, as much of the depleted coating as possible, without affecting the base material under the coating unless this also has been oxidized. Therefore, it is highly advantageous to prepare the areas requiring local MCrAlY repair by cleaning using any conventional means such as local chemical etching, grit blasting, grinding or other abrasive methods. This will ensure a long lasting bond of the newly applied coating material to the substrate.
Another advantage comes from the fact that the method is also applicable even when a ceramic coating is existent. The ceramic coating, which is on top of the MCrAlY-coating, can be removed with any possible means before applying the steps of the method of the invention and the article is re-coated with a ceramic coating thereafter.
In another embodiment the aluminising and/or chromising takes place before the local repair of MCrAlY which is still possible to fulfil the same desired effect.
A further advantage would be that the problem of plugging the cooling holes with sprayed coating would be avoided. Since cooling holes offer local protection from high temperatures, the coating would not require local replacement close to the cooling holes. Thus, the coating could be locally replaced in the hot areas and then the entire blade aluminised without plugging the cooling holes with sprayed coatings. In any case it is possible to mask the cooling holes during the local coating repair or aluminising method according to the invention to avoid a reduction of the size of the cooling holes during the proposed method.
Of course, an article comprising an inner and an outer surface with a MCrAlY-coating will be aluminised and/or chromised at the said inner and at the said outer surface.
The enrichment with Al and/or Cr within the MCrAlY-coating is optimised when the aluminising and/or chromising is followed by a diffusion heat treatment. Alternatively a xe2x80x9chigh activityxe2x80x9d aluminising can be used so that Al is deposited not only at a surface layer of the MCrAlY-coating, but diffuses into the MCrAlY-coating. Preferably the aluminising takes place with a gas phase method.
In another advantageous embodiment, the local repair of the MCrAlY-coating takes place with a corrosion resistant coating containing a high amount of Cr. With that embodiment the corrosion resistance is enhanced at those areas most vulnerable thereby increasing the overall life time of article.
An article as it is claimed can possibly be a blade or a vane or any other part of a gas turbine engine coated with a MCrAlY-coating and exposed to a high temperature. environment.